Changed the default value of --pretty from False to True to provide
visually nicer output by default. Users can still disable it with
--no-pretty if they prefer more concise error messages.

Updated defaults in:
- mypy/options.py: Changed self.pretty = False to True
- mypy/main.py: Changed default=False to default=True for --pretty flag
- docs/source/command_line.rst: Added note about default and --no-pretty

Fixes python#19108

- Use literal markup for --no-pretty in docs to avoid unknown option warning
- Set pretty=False in test infrastructure so existing test expectations
  remain valid when --pretty is not explicitly passed as a flag

Fix add_invertible_flag to always use store_true for the flag and
store_false for its inverse, regardless of the default value. The
previous logic inverted the actions when default=True, causing
--pretty to disable pretty mode and --no-pretty to enable it.

Update all test harnesses to explicitly disable pretty mode for tests
that don't opt into it, preventing source code snippets from appearing
in test output that expects non-pretty format.

Restore the original add_invertible_flag logic where the argparse action
depends on the default value. The previous commit incorrectly simplified
this to always use store_true/store_false, which broke all 7 flags with
default=True (--no-namespace-packages, --no-warn-no-return,
--no-implicit-reexport, --no-color-output, --no-error-summary, etc).

The --pretty default is now controlled solely through Options.pretty=True
in options.py, keeping default=False in add_invertible_flag so the flag
semantics work correctly (--pretty -> store_true, --no-pretty -> store_false).

Also fix ruff PIE810 by merging startswith calls into a tuple.

The previous logic incorrectly appended --no-pretty after -- for all
dmypy commands (like dmypy check), which caused --no-pretty to be
treated as a filename. It also appended "-- --no-pretty" to dmypy run
commands without --, which argparse rejected.

Fix by:
1. Only injecting --no-pretty for dmypy run/start (not check/recheck)
2. For dmypy run/start without --, properly parse out dmypy-specific
   flags and reconstruct the command with -- separator so --no-pretty
   goes into the mypy flags portion.